1. Field of the Invention
The invention relates to a process for producing an electric heating element in which metallic heating conductors are embedded between ceramic insulating layers, and, as power supply leads and power outlet leads, contact recesses in the ceramic insulating layers are filled with an electrically conductive composition.
2. Description of Related Art
In CERAMIC BULLETIN 60, p. 540 ff (1981), Otsuka et al. describe how components of ceramic materials containing mostly aluminum oxide or aluminum nitride can be metallized with high-melting metals such as tungsten or molybdenum, how this metallization can then be covered by a further layer of green ceramic and the composite can then be sintered to form the material. The sheet technique is particularly suitable for this purpose.
Components thus produced can be used primarily in the fields of electronics and electrical engineering. Thick-walled and large heating elements are known in various forms. However, with the increasing miniaturization in electronics and electrical engineering, problems occur in production and use.
At high heating power, the materials used have to be resistant to high temperatures without the heating elements being destroyed by overloading (burning through).
Use as heating element is thus possible where the action of an electric current generates large amounts of heat preferentially at those points in the metallization pattern which have a high resistance. Here, in the high-temperature heating range, temperatures can be generated which are so high that use of ceramic materials containing glass phases (glass content &gt;5% by weight) results in flowing of the glass phase in the ceramic. If the distance of a metallic heating strip conductor to the outer surface of the ceramic in the multilayer is here very small, in particular less than 0.4 mm, air can penetrate through the zones depleted in glass phase, i.e. the zones having a particularly high temperature, to the metallic conductor comprising tungsten or molybdenum and destroy this power conductor by oxidation. For this reason it is advantageous to use a ceramic having a low proportion of glass phase.
From Otsuka et al. it is likewise known that metallization pastes without proportions of glass or glass formers have little adhesion to such ceramic materials low in glass phase. It is known that addition of glass can greatly increase the adhesion, but such pastes have a high electrical resistance which is a disadvantage in very fine structures.